Fluorescence resonance energy transfer (FRET) has emerged as a powerful tool to the study of protein-protein interactions, such as receptor-ligand binding. However, the application of FRET to the study of G protein-coupled receptors (GPCRs) has been limited by the method of labeling receptor with fluorescence probes. Here we described a novel time-resolved (TR)-FRET method to study GPCR-ligand binding by using human complement 5a (C5a) receptor (C5aR) as a model system. Human C5aR was expressed in human embryonic kidney 293 cells with a hemagglutinin (HA) epitope at the N-terminus. Purified human C5a was labeled with terbium chelate and used as the fluorescence donor. Monoclonal anti-HA antibody conjugated with Alexa Fluor 488 was used as the fluorescence acceptor. Robust FRET signal was observed when the labeled ligand and C5aR membrane were mixed in the presence of the conjugated anti-HA antibody. This FRET signal was specific and saturable. C5a binding affinity to C5aR measured by the FRET assay was consistent with the data as determined by competition binding analysis using radiolabeled C5a. The FRET assay was also used to determine affinity of C5aR antagonists by competition binding analysis, and the data are similar to those from radioligand binding studies. Compared to the commonly used radioligand binding assay, this TR-FRET-based assay provides a nonradioactive, faster, and sensitive homogeneous assay format that could be easily adapted to high-throughput screening. The principle of this assay should also be applicable to other GPCRs, especially to those receptors with peptide or protein ligands.